Vehicle interiors are continually fed with outside air drawn in by the vehicle's ventilation, heating and cooling system, generally called an HVAC system. Interior air must be continually vented, therefore, to maintain an interior pressure balance, which is typically done through a one way valve. Backflow through the valve is undesirable, because backflow air will not be conditioned to the desired temperature and humidity, and may carry particulates and dust.
Ventilation valves can be electro mechanical devices that respond actively to pressure differentials, but these are expensive and complex. A more common valve is a passively acting one way device that vents excess interior pressure, but closes off automatically to prevent backflow. One such valve is a simple duck bill device, as shown in U.S. Pat. No. 4,691,623 to Mizusawra. Duck bill valves open and close quickly, but have a very limited opening area and venting capacity. A more common type of passive valve uses one or two thin, flexible rubber flaps that sit on an inclined frame, with the flaps' own weight holding them down to cover a large area vent opening. Excess interior pressure easily lifts the flaps to allow quick venting through the large vent opening, while the flaps own weight quickly closes it once a pressure balance is restored. An example of such a flap type ventilation valve may be seen in co-assigned U.S. Pat. No. 4,781,106. Because of the large potential venting area, a flap valve can work quickly. The extreme flexibility of the thin flap requires that support webs be provided to prevent it from getting sucked back through the vent opening and sticking. The support webs cross the vent opening like a window frame, and effectively divide it up into numerous smaller area openings, as may be seen in FIG. 1 of the last cited patent. While the support webs are needed, they threaten seal integrity whenever the flap faces a negative pressure, which causes it to bulge inwardly. At the T shaped junctions where the webs intersect the edge of the vent opening, the lip of the flap can be lifted or puckered locally, allowing some back flow leakage.
The problem of back flow with thin rubber flaps has been recognized, as in recently issued U.S. Pat. No. 5,085,132. The solution proposed there is simply to provide a filter to catch particulates carried in by the backflow air. This, of course, entails extra cost, and does not prevent the ingress of air that is too hot or cold, or too wet. It would be far preferable to deal with the root cause of backflow leakage per se, especially if it could be done at no extra cost.